Add langgraph agentic files to repo

[indrayudd]

Add 3 levels of langgraph agents:

• Simple agentic loop on mathematical expressions (to understand how agents work, beginner friendly)
• Simple v0 agent that does basic operations agentically on timeseries data (we build up from here)
• Attempt at recreating InsightPilot (buggy, needs to be improved, we can optionally build from here too)